Fluid working machines include fluid-driven and/or fluid-driving machines, such as pumps, motors, and machines which can function as either a pump or as a motor in different operating modes. Although the invention will be illustrated with reference to applications in which the fluid is a liquid, such as a generally incompressible hydraulic liquid, the fluid could alternatively be a gas.
When a fluid working machine operates as a pump, a low pressure manifold typically acts as a net source of fluid and a high pressure manifold typically acts as a net sink for fluid. When a fluid working machine operates as a motor, a high pressure manifold typically acts as a net source of fluid and a low pressure manifold typically acts as a net sink for fluid. Within this description and the appended claims, the terms “high pressure manifold” and “low pressure manifold” refer to manifolds with higher and lower pressures relative to each other. The pressure difference between the high and low pressure manifolds, and the absolute values of the pressure in the high and low pressure manifolds will depend on the application. For example, the pressure difference may be higher in the case of a pump which is optimised for a high power pumping application than in the case of a pump which is optimised to precisely determine the net displacement of fluid, for example, a pump for dispensing a metered amount of fluid (e.g. a liquid fuel), which may have only a minimal pressure difference between high and low pressure manifolds. A fluid working machine may have more than one low pressure manifold.
Fluid working machines are known which comprise a plurality of working chambers of cyclically varying volume, in which the displacement of fluid through the working chambers is regulated by electronically controllable valves, on a cycle by cycle basis and in phased relationship to cycles of working chamber volume, to determine the net throughput of fluid through the machine. For example, EP 0 361 927 disclosed a method of controlling the net throughput of fluid through a multi-chamber pump by opening and/or closing electronically controllable poppet valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the pump and a low pressure manifold. As a result, individual chambers are selectable by a controller, on a cycle by cycle basis, to either displace a predetermined fixed volume of fluid or to undergo an idle cycle with no net displacement of fluid, thereby enabling the net throughput of the pump to be matched dynamically to demand. EP 0 494 236 developed this principle and included electronically controllable poppet valves which regulate fluid communication between individual working chambers and a high pressure manifold, thereby facilitating the provision of a fluid working machine functioning as either a pump or a motor in alternative operating modes. EP 1 537 333 introduced the possibility of part cycles, allowing individual cycles of individual working chambers to displace any of a plurality of different volumes of fluid to better match demand.
Fluid working machines of this type require rapidly opening and closing electronically controllable valves capable of regulating the flow of fluid into and out of a working chamber from the low pressure manifold, and in some embodiments, the high pressure manifold. Some aspects of the invention aim to provide improved valve assemblies suitable for regulating the flow of fluid into and out of the working chamber of fluid working machines of this type. However, the valve assemblies of the present invention are applicable to other types of fluid working machine.
Some aspects of the present invention address the problem of opening a face seating valve, such as a poppet valve, against a pressure differential, to regulate the supply of fluid from a high-pressure manifold to a working chamber of a fluid working machine. This is technically difficult because, in a face seating valve, the fluid pressure acts over the seating area to create a large closing force. Accordingly, it is difficult to provide a face seating valve for regulating the supply of fluid from a high-pressure manifold to a working chamber of a fluid working machine which is capable of opening against a significant pressure differential and which also is also capable of opening quickly (ideally within a few milliseconds) whilst minimizing energy consumption.
GB 2,430,246 (Stein) discloses a valve assembly which is suitable for regulating the supply of fluid from a high-pressure manifold to a working chamber of a fluid working machine. The valve assembly comprises a primary valve, a secondary valve, an electromagnet and an armature (referred to as a moving pole). The primary valve comprising a face-seating primary valve member and a primary valve seat. The secondary valve is integral to the primary valve and includes a secondary valve member which is moveable between a sealing position and an open position in which a path is provided through the secondary valve for fluid to flow between opposite sides of the primary valve member to reduce the pressure difference across the primary valve member. Thus, the secondary valve, which has a much smaller surface area than the primary valve, can be opened even when there is a substantial pressure difference across the primary valve member. The working chamber is effectively a closed volume, and so fluid can flow through the secondary valve to equalise the pressure on either side of the primary valve member, thereby facilitating the opening of the primary valve.
In the arrangement disclosed in GB 2,430,246, the armature is slidable along a path extending between a first position and a second position, which is closer to the electromagnet. The armature is resiliently coupled to the primary valve member by spring 12. The armature is also coupled to the secondary valve member, in the embodiment of FIG. 5, or integral with the secondary valve member, in the embodiment of FIG. 1. Thus, movement of the armature is fixedly linked to movement of the secondary valve member, and so the secondary valve is opened by the initial movement of the armature, when the electromagnet is first switched on. In practice, we have found that it is difficult to produce a sufficient force from the action of the electromagnet on the armature, without excessive energy consumption. This is especially relevant in the embodiment of FIG. 5 of GB 2,430,246, where the secondary valve is also a face seating valve having the same orientation as the primary valve and therefore also subject to significant closing forces due to the pressure differential between the inlet and the outlet. Accordingly, some aspects of the present invention aim to provide an improved valve assembly, which could open against a greater pressure difference, or more quickly, or with a less energy consumption than the valve assemblies disclosed in GB 2,430,246.
Another technical problem which can arise with valve assemblies including electronically actuatable face seating valves (such as poppet valves), for regulating the supply of fluid into the working chamber of a fluid working machine, relates to the requirement to hold the face seating valve open whilst fluid is flowing through the valve. Bernoulli effects (kinetic energy related pressure drop) and surface friction arising from the flow of fluid past the face seating valve element (e.g. a poppet head) can exert a substantial force on the face seating valve element. Thus, it may be necessary to continue to supply a substantial amount of power to the electromagnet to keep the face seating valve open, or this effect may limit the maximum flow rate through the valve. In the valve assembly disclosed in GB 2,430,246, the face seating valve is held open by a spring. In practice it is extremely difficult for this spring to provide enough force to hold the valve open against the Bernoulli and surface friction forces. Accordingly, some embodiments of the invention address the problem of holding open electromagnetically actuatable face seating valves while fluid flows through the valves from a low or high pressure manifold to a working chamber of a fluid working machine, or in the reverse direction.